Constant current source and battery charger

ABSTRACT

A constant current battery charger includes a bucking regulator comprising a power switch transistor, a diode, an inductor and a voltage sensitive switch with adjustable delay for controlling the power switch transistor. The hysteresis or delay is obtained with a Schmitt trigger and the effect is a stable pulse width modulator in which the duty cycle of the power switch transistor is varied. A current foldback circuit includes a zener diode coupled to the battery for changing the sensitivity of the switch to substantially reduce charging current when a battery voltage corresponding to a fully charged state is reached.

This applicationis a continuation-in-part of application Ser. No.127,745, filed 12/2/87, now U.S. Pat. No. 4,829,259, issued May 9, 1989,and is related to application Ser. No. 272,194, filed 11/16/88, which isa division of Ser. No. 127,745.

BACKGROUND OF THE INVENTION AND PRIOR ART

This invention is directed to a novel battery charger utilizing aconstant current source and speed up circuit and which includesautomatic charging current reduction as the battery becomes fullycharged.

Battery chargers using high speed pulse width modulation (PWM) switchingof constant current sources are well-known in the art for supplyingcurrent for charging batteries. High speed transistor switchingcircuits, particularly those used with single-ended power supplies, arelimited by the time required to remove base current from the switchtransistor. In double-ended power supplies, i.e., those providingpositive and negative operating potentials, the problem of rapidlyremoving base current is simplified as compared with single-endedsupplies where one side of the supply is held at ground potential.

A speed up circuit, claimed in application Ser. No. 272,194 above, isused in the present invention battery charger to rapidly remove the basedrive to the switch transistor and thereby result in a dramaticenhancement of its switching characteristics. The decrease in switchingtime enables a significant reduction in heat dissipation in thetransistor and permits utilization of a much smaller heat sink.

In many applications, such as in portable television receivers andcomputers where the batteries comprise a very large part of the overalldevice cost, it is desirable to use rechargeable battery packs.Rechargeable battery packs generally have nickel cadmium batteries, theuseful lives of which are highly dependent upon the manner in which theyare recharged. It is common for a manufacturer to guarantee a batterypack for a minimum number of discharge and charge cycles, provided thatthe batteries are recharged under controlled conditions. Nickel cadmiumbatteries are best charged either with a low continuous current or witha larger constant current until a certain battery temperature isattained. To do otherwise can have an adverse effect on their usefullives. Consequently, the equipment used to recharge the battery packmust be carefully designed. As a battery recharging circuit is oftenbuilt into the powered device, space and weight are at a premium and ahigh efficiency recharging circuit is desirable.

The present invention is directed to a switching type recharging circuitfor supplying a constant RMS current to a battery pack and includes anautomatic current reduction or "foldback" circuit for automaticallyreducing the charging current as the battery reaches a fully chargedstate. For the battery pack used, the manufacturer specifies either acharging rate of 1.2 amps until a case temperature of 55° C. isattained, or a continuous charge of 200 milliamps. The circuitconstructed in accordance with the invention accomplishes this with veryhigh efficiency, on the order of 84%, without a speed up circuit. Withthe speed up circuit of the copending application, an efficiency inexcess of 90% is attained. Further, the current supplied to the batterychanges less than 1% from discharge to full charge and thereafter idlesat the recommended minimum level.

OBJECTS OF THE INVENTION

A principal object of the invention is to provide a novel batterycharger.

Another object of the invention is to provide a highly efficient,constant current battery charger with automatic current foldback.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the invention will be apparentupon reading the following description in conjunction with theaccompanying drawings, in which like reference numbers indicate likeelements:

FIG. 1 is a schematic diagram of a battery charger and constant currentsource constructed in accordance with one aspect of the inventiondisclosed in the above-identified patent;

FIG. 2 is a series of waveforms depicting the duty cycle and currentflow characteristics of the circuit of FIG. 1;

FIG. 3 represents a combined block diagram of a transistor switchincorporating a speed up circuit constructed in accordance with anotheraspect of the invention disclosed in the above patent; and

FIG. 4 is a battery charger of FIG. 1 utilizing the speed up switch ofFIG. 3 and including automatic current foldback of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a source of DC voltage 10 is coupled to the emitterof a PNP power switch transistor 12, the collector of which is connectedto the junction of the cathode of a diode 14 and one terminal of aninductor 16. The other terminal of the inductor 16 is coupled to thepositive terminal of a battery pack 18, the negative terminal of whichis returned to ground through series connected current detectorresistors 20 and 20a. Diode 14 has its anode connected to ground. DCvoltage source 10 is also connected to the base of power switchtransistor 12 through a resistor 22 and to a voltage divider consistingof three series-connected resistors 32, 34 and 36. The base of powerswitch transistor 12 is coupled through a resistor 24 to the collectorof an NPN transistor 26, the emitter of which is connected to ground,through a movable tap 30 on a potentiometer 28, and the base of which isconnected to the junction of resistors 34 and 36. The emitter of NPNtransistor 26 is also connected to the emitter of another transistor 38having its collector connected to the junction of resistors 32 and 34and its base connected through a resistor 40 to the junction of battery18 and resistors 20 and 20a. Under normal conditions, resistor 20a isshorted out by a connection to ground through a contact 29. Transistors26 and 38 are connected to form a Schmitt trigger for providinghysteresis or delay in the ON and OFF cycling of power switch 12. Theletters A, B and C will be discussed in connection with FIG. 4.

A temperature sensor 25 is arranged with respect to battery pack 18 tomeasure the case temperature. This may be accomplished in any number ofwell-known ways. Sensor 25 is coupled to a temperature relay 27 whichoperates a contact 29 when the temperature measured by sensor 25indicates that the limit temperature for the battery pack 18 has beenreached. Contact 29 is opened by relay 27 to remove the short circuitaround resistor 20a.

In operation, the circuit forms an oscillator, the ON time of which iscontrolled by the connected load, i.e., the state of charge of thebattery pack 18. The battery charge is reflected in its terminal voltageand, in the preferred embodiment, may range from 0-34 volts. Arequirement of the circuit is that the potential applied by DC voltagesource be approximately 4 volts higher than the highest terminal voltageattainable by battery 18. When the circuit is activated, for example, bya switch (not shown), transistor 26 and power switch transistor 12 aredriven conductive. This is due to the forward bias established byresistors 32, 34 and 36 which drives transistor 26 conductive, therebyturning on power switch transistor 12. The collector-emitter current oftransistor 26 flows through potentiometer 28 which develops a positivepotential at the emitter of transistor 38. Conduction of power switchtransistor 12 produces an increasing current ramp through inductor 16,battery pack 18 and current detector resistor 20 (resistor 20a isshorted). The current ramp increases at a steady rate until the voltagedeveloped across current detector resistor 20 exceeds the voltage acrosspotentiometer 28 by 0.7 of a volt (the base-emitter conduction potentialfor transistor 38). Transistor 38 conducts and immediately shorts outthe base current drive to transistor 26 by effectively connecting thejunction of resistors 32 and 34 to the emitter of transistor 26.Transistor 26 ceases conduction and power switch transistor 12 is drivennon-conductive. At this time, the energy stored in the field of inductor16 is released as the field collapses and a decreasing ramp of currentflows through battery pack 18, resistor 20 and diode 14.

Resistor 32 is much larger than resistor 24 and consequently, thecurrent flow through potentiometer 28 is greatly reduced. This path isfrom DC voltage source 10 through resistor 32 through thecollector-emitter junction of transistor 38 through potentiometer 28 toground. The prior path, it will be recalled, was through resistor 22 and24 and the collector-emitter path of transistor 26. Consequently, thepotential across potentiometer 28 drops significantly (on the order of500%) which permits transistor 38 to remain conductive despite thedecreasing voltage across resistor 20. Therefore, power switchtransistor 12 is held in cutoff while the energy in the collapsing fieldof inductor 16 continues to cause current flow in battery 18 andresistor 20. When the voltage across resistor 20 drops from its peakvalue by 500%, transistor 38 ceases conduction and permits transistor 26(and power switch 12) to conduct again. Resistor 40 is for currentlimiting purposes only. When the potential across resistor 20 issufficiently low, transistor 38 no longer can conduct and the cyclerepeats with transistor 26 being driven conductive and again turning onpower switch transistor 12. The oscillatory cycle continues with theduty cycle (and frequently of operation) of transistor 12 being changedas the potential across battery pack 18 increases during charging.

Reference to FIG. 2 indicates this relationship. The curves are arrangedto show voltage across the power switch and battery current for batteryterminal voltages of 32 V, 18 V, 5 V and 1V. The upper ones of thecurves A0-A3 show the voltage across the emitter-collector of powerswitch transistor 12 and the lower ones of the curves B0-B3 depicts thecurrent flow into battery pack 18. Beginning with the lowest curve setA3 and B3, representing a battery having a voltage across its terminalof one volt, it will be seen that the ON time of the power switchtransistor is very short whereas its OFF time is quite long. As thebattery terminal voltage is raised, the OFF time of switch transistor 12gets shorter and the ON time gets longer. Also the frequency ofoscillation changes. It can readily be seen that the circuit isself-regulating and delivers a constant RMS current to the load under awide range of varying load conditions. 55° C.), contact 29 will removethe short circuit around resistor 20a and add sufficient resistance tokeep transistor 38 conductive for a much longer period. The addedresistance is selected to maintain a 200 milliamp constant chargecurrent.

Referring to FIG. 3, a single-ended DC potential source 10 is coupled tothe emitter of PNP switch transistor 12 that has its collector connectedto a load circuit 13 instead of to diode 14 and inductor 16 as inFIG. 1. Load circuit 13 is coupled to a control voltage source 15 thatsupplies the base of an NPN trigger transistor 26. Transistor 26controls the turn on and turn off of switch transistor 12. The collectorof trigger transistor 26 is connected to the base of switch transistor12 through a resistor 24. The emitter-base junction of switch transistor12 is connected across the collector-emitter junction of an NPN shortingtransistor 17. The base of shorting transistor 17 is connected to thejunction of the cathode of a diode 19 and a resistor 21. The other endof resistor 21 is connected, through a small capacitor 23, to thecollector of trigger transistor 26 and the anode of diode 19 isconnected to the emitter of shorting transistor 17.

The elements comprising shorting transistor 17, diode 19, resistor 21and capacitor 23 take the place of the normal emitter-base resistor 22of switch transistor 12 (as seen in FIG. 1). Assuming trigger transistor26 is conductive, switch transistor 12 is conductive and a voltage dropexits across resistor 24. The collector of trigger transistor 26 is atsubstantially ground potential and capacitor 23 is charged in thepolarity indicated. Shorting transistor 17 is non-conductive.

In response to an appropriate negative going potential signal fromcontrol voltage source 15, trigger transistor 26 is drivennon-conductive. As its collector potential begins to rise, e.g., triggertransistor 26 begins to turn off, capacitor 23 discharges throughresistor 21, the base-emitter junction of shorting transistor 17 andresistor 24. This occurs very rapidly and a large peak current of shortduration, sufficient to drive shorting transistor 17 into saturation, isdeveloped. Shorting transistor 17 responds by immediately shorting outthe base-emitter junction of switch transistor 12 and quickly drawingaway its base charge. The rapid depletion in base charge results in avery fast cutoff transition time. When the control voltage from controlvoltage source 15 becomes positive going, trigger transistor 26 is againdriven conductive. An initial conductive circuit in parallel withresistor 24 is established through diode 19, resistor 21, capacitor 23and the saturated collector-emitter junction of transistor 26 to ground.Current flow in this circuit is from the base of switch transistor 12and is typically ten times greater than the current which flows throughresistor 24. This higher than normal current is what accounts for theincreased turn-on speed of switching transistor 12. As capacitor 23charges, the current through the base of switch transistor 12 drops invalue. When capacitor 23 is completely charged, the only current flowingthrough the base of switch transistor 12 is that which flows throughresistor 24.

The increase in turn on/turn off speed of switch transistor 12 in abattery charger circuit constructed in accordance with FIG. 1 had adramatic effect on circuit efficiency. The switching time decreased from500 nanoseconds, with the base-emitter resistor embodiment shown in thatapplication, to 20 nanoseconds--a 25-fold decrease in switching time.Similar increases in switching speed have been observed when theinventive speed up circuit was employed in a television high voltageswitch application. The increased switching speed of the battery chargerproduced about a 10% increase in efficiency and reduced the need forheat sinking.

Referring to FIG. 4, a battery charger circuit arrangement substantiallyas shown in FIG. 1 incorporates the high speed switching circuit of FIG.3. As indicated by the letters A, B and C on FIG. 1 and FIG. 4, the highspeed switching circuit arrangement of FIG. 4 may be substituted for theemitter-base resistor 22 of switch transistor 12 in FIG. 1. Theautomatic current foldback feature is provided by the connection of thecathode of a zener diode 50 to the junction of inductor 16 andthepositive terminal of battery 18. The anode of zener diode 50 isconnected to ground through a filter capacitor 52 and to the anode of adiode 54. The cathode of diode 54 is connected through a resistor 56 tothe base of transistor 38. Additionally, potentiometer 28 is replaced byan adjustable resistor 28' and a resistor 29 in the base circuit oftransistor 26 to provide limited adjustment of the emitter resistance oftransistor 26. The operation of the battery charger portion of FIG. 4will be briefly described.

As mentioned, the circuit forms an oscillator, the ON time of which iscontrolled by the connected load, i.e., the state of charge of thebattery 18. When the circuit is turned on, transistor 26 and powerswitch transistor 12 are driven conductive. Resistors 28' and 29 developa positive potential at the emitter of transistor 38. Without thefoldback circuit being considered, the increasing current ramp throughinductor 16, battery 18 and current detector resistor 20, causes thevoltage developed across current detector resistor 20 to exceed thevoltage across resistors 28' and 29 by 0.7 of a volt. Transistor 38conducts and immediately shorts out the base current drive to transistor26, which ceases conduction to drive power switch transistor 12non-conductive.

With the automatic foldback circuit, operation is modified as follows.When the voltage at the positive terminal of battery 18 reaches apredetermined level, namely the breakdown voltage of zener diode 50, thebase of transistor 38 becomes forward biased. Transistor 38 conducts andreduces the base drive for transistor 26 which controls the duty cycleof switch transistor 12. For the battery used in the describedembodiment of the invention, one and one-half amperes of chargingcurrent is delivered until 23 volts is developed across the battery. At26 volts across the battery, the charging current is reduced to about1.3 amperes. At 27 volts, the current is further diminished to 0.6amperes and at 27.1 volts, it is reduced to the recommended 0.20amperes. This is accomplished automatically by the foldback circuit.

Resistor 32 is much larger than resistor 24 and consequently, thecurrent flow through adjustable resistor 28' and resistor 29 is greatlyreduced. This path is from DC voltage source 10, through resistor 32,through the collector-emitter junction of transistor 38, throughadjustable resistor 28' and resistor 29, to ground. The prior path, itwill be recalled, was through resistor 22 and 24 and thecollector-emitter path of transistor 26. Consequently, the potentialacross resistors 28' and 29 drops significantly (on the order of 500%)which permits transistor 38 to remain conductive despite the decreasingvoltage across resistor 20. Therefore, power switch transistor 12 isheld in cutoff while the energy in the collapsing field of inductor 16continues to cause current flow in battery 18 and resistor 20. When thevoltage across resistor 20 drops 500% from its peak value, transistor 38ceases conduction and permits transistor 26 (and power switch 12) toconduct again. Resistor 40 is for current limiting purposes only. Whenthe potential across resistor 20 is sufficiently low, transistor 38 nolonger can conduct and the cycle repeats, with transistor 26 beingdriven conductive and again turning on power switch transistor 12. Theoscillatory cycle continues with the duty cycle (and frequency ofoperation) of transistor 12 being changed as the potential acrossbattery 18 increases during charging.

The speed up circuit used with the present invention operates asdescribed above in connection with FIG. 3 to accelerate the switchingtransitions of switch transistor 12 to thereby obtain furtherimprovements in switching speed and efficiency.

What has been described is a novel battery charging circuit whichincludes an automatic foldback circuit for restricting charging current.It is recognized that numerous changes in the described embodiment ofthe invention will be apparent to those skilled in the art withoutdeparting from its true spirit and scope. The invention is to be limitedonly as defined in the claims.

What is claimed is:
 1. A battery charger operated from a single-endedpower supply comprising:pulse width modulation means including atransistor power switch, a diode and an inductor for supplying currentto a battery; means for monitoring current flow to said battery; triggermeans for controlling the frequency and duty cycle of said pulse widthmodulation means responsive to said monitoring means; a speed up circuitfor said switch transistor comprising a shorting transistor coupledacross the base-emitter circuit of said switch transistor and meansresponsive to said trigger means for driving said shorting transistorconductive; current foldback means including zener diode sensing meansfor monitoring the voltage of said battery; and means for changing saidmonitoring means to substantially reduce current flow to said batterywhen a battery voltage corresponding to a fully charged state isreached.
 2. A charger in accordance with claim 1 wherein said triggermeans includes a Schmitt trigger arrangement coupled between saidmonitoring means and said power switch transistor for introducing adelay in controlling said duty cycle.
 3. The battery charger of claim 2wherein said zener diode sensing means comprises a zener diode in aseries resistive path between said battery and said Schmitt trigger.